In ground drilling it is customary to detachably fix a drill bit to a lower end of a drill string of a ground drill and rotate the drill string to effect drilling of a hole in the ground by the drill bit. A reamer is usually connected between the lower end of the drill string and the drill bit to ream the circumferential wall of a hole being drilled. The drill string is formed by screwing individual drill rods together. Drill rods usually come in fixed lengths of 1.5, 3 or 6 meters. As the drill progresses into the ground additional drill rods are screwed into the upper end drill string.
During drilling it will be necessary to replace the drill bit and reamer either as a result of dulling of the drill bit or due to variations in the sub strata. Although the drill bit must be replaced more often (usually at least six times more often) than the reamer.
In order to replace a drill bit or reamer the entire drill string must be pulled out of the ground rod by rod, the drill bit replaced, and the drill string reassembled, rod by rod as it is relowered into the ground to continue drilling. The need to fully withdraw, disassemble and reassemble the drill string when changing the drill bit/reamer is a slow and costly exercise, with the cost increasing as hole depth increases and the drill string becomes longer.
Several attempts have previously been made to overcome this problem at least insofar as drill bits are concerned by use of retractable drill bits which releasably engage the lower end of the drill string and can be disengaged and retracted through the drill string for changing while the drill string remains in situ, thereby avoiding the need to withdraw the drill string from the hole. However, these attempts have not proven to be commercially successful for various reasons including: being extremely complicated in design or application thereby resulting in a large number of failure modes and/or being to costly to manufacture or maintain in an operational state; being prone to fouling due to drilling fluid and contaminants burring or jamming segments of the drill bit; misalignment of drill bit segments upon engagement with the drill string; reduction in diameter of the core sample due to fixing of the drill bit to an inner tube of the drill string; reduction in penetration rate; and breaking of individual segments of the drill bit.